The next generation of solid-state lighting is seeking to provide advances in brightness, efficiency, color, purity, packaging, scalability, reliability and reduced costs. The creation of light emitting devices from silicon based materials, upon which the modern electronic industry is built, has been the subject of intensive research and development around the world. To overcome the inherent low efficiency of light emission from indirect bandgap materials, such as bulk silicon and other group IV semiconductor materials, extensive research has been directed to nanostructures, i.e. nano dots, nanocrystals and superlattice structures, and materials comprising silicon nanocrystals and/or other luminescent centres, such as rare earth ions, in a suitable host matrix. Structures comprising the latter materials are disclosed in related copending U.S. published applications publication No. 20080093608 entitled “Engineered structure for solid state light emitters” and No. 20080246046 entitled “Pixel structure for a solid state light emitting device”, and references cited therein.
Rare earth elements have been employed in various forms of illumination sources for decades. It is well known that the rare earth elements can emit infrared, visible and ultraviolet light when exited optically or electrically. For example, rare earth doped glass has been used for many years as phosphors, in solid-state lasers, and fibre based optical amplifiers. In these devices, the rare earth dopants in a glass host matrix material act as luminescent centres that absorb and emit radiation at specific wavelengths that are determined by the valence state of the ion, and are, to a large degree, independent of the host in which the ions sits. The colour of the emission is characteristic of the particular rare earth species. For example, erbium and terbium emit in the green, europium emits in the red, cerium and thulium emit in the blue regions of the visible spectrum. By using a selection of different rare earth elements, the individual colours can be combined in predetermined ratios to emit various colours, or white light of a desired colour rendering index (CRI).
Rare earths have been used in gaseous form where they are excited through a high voltage gas discharge, or as solid phosphors, in which they are excited by another light source operating in the deep blue or ultraviolet.
More recently various laboratories have looked at applying rare earths to solid-state lighting sources, and particularly white light emitters using electroluminescent devices fabricated using silicon or other Group IV semiconductors. For example, U.S. Pat. No. 7,122,842 to Hill, entitled “Solid state white light emitter and display using same” discloses a white light emitter comprising layers of rare earth doped group IV semiconductor nanocrystal material, wherein each layer is doped with a different rare earth dopant to collectively emit visible light. An article entitled “Bright green electroluminescence from Tb3+ in silicon metal oxide semiconductor devices” by J. M. Sun et al., Appl. Phys. Lett. 97, 123513 (2005) discloses visible light emission from terbium doped silicon dioxide.
Methods for deposition of rare earth doped group IV nanocrystal materials are disclosed in U.S. Pat. No. 7,081,644, to Hill, entitled “Doped Semiconductor Powder and Preparation thereof”. Rare earth containing oxides, or rare earth doped oxides, can be formed by any of a number of techniques such as ion implantation, chemical vapour deposition, physical vapour deposition (i.e. sputtering), spin-on (sol gel) techniques, beam deposition, laser deposition, or any of a large number of similar chemical or physical deposition techniques that are generally well known in the thin film or semiconductor technology fields.
The generation of light from electroluminescent solid state light emitting devices (EL devices), as generally described in this application, and related copending applications, is based on applying energy from an electric field to a light emitting structure including an active region or emissive layer. Active layers may comprise a wide band gap semiconductor or dielectric e.g. silicon nitride, silicon dioxide, or GaN, including luminescent centres, such as semiconductor nanocrystals and/or luminescent rare earth species. It is important to deliver a minimum and controlled amount of electron energy for excitation of luminescent centres in an active light emitting layer in the device. If the energy of incident electrons is too low there will be no light emission possible. On the other hand, if the electrons possess too much energy there will be light emission but excess energy will be carried away in the form of heat, which reduces efficiency. Furthermore, hot electrons can be responsible for damage to the host matrix, result in charging, and ultimately contribute to breakdown and failure of the device under bias.
There has been particular interest in rare earth doped nanoparticle materials because silicon nano-particles act as classical sensitizer atoms that absorb incident photons or electrons and then transfer the energy to the rare earth ions, which then fluoresce in the infrared or visible wavelength ranges, with several advantages compared to the direct fluorescence of the rare earth. First, the absorption cross-section of the silicon nano-particles is larger than that of the rare earth ions by more than three orders of magnitude. Second, as excitation occurs via an Auger-type interaction or via a Forster transfer process between carriers in the silicon nanoparticles and rare earth ions, incident photons need not be in resonance with one of the narrow absorption bands of the rare earth luminescent centers. Unfortunately, existing approaches to developing such silicon nano-particle materials have only been successful at producing very low concentrations of the rare earth element, which are not sufficient for many practical applications. Also, silicon nano-particles formed by such techniques generally have a relatively narrow distribution of photo-luminescent (PL) wavelength or energy despite the broad size distribution, i.e. the observed energies are not as high as expected from the quantum confinement of the nanocrystals. The reduced nano-particle excitation energy affects the efficiency of energy transfer from conducting electrons when these structures are electrically powered, thereby limiting the efficiency of light generation from such films.
As described in detail copending published patent application No. 20080093608 since the excitation energy and emission wavelength of nanoparticles or nanocrystals is dependent on the nanoparticle size and size distribution, the thickness of each active layer comprising nanoparticles may be selected to control the size and uniformity of nanoparticles. In practice, however, careful control of deposition parameters, layer thickness, and thermal treatments is needed to control the size, uniformity and passivation of nanocrystal layers to obtain a desired emission wavelength and excitation energy, otherwise significant emission may be observed from lower energy interfacial states, resulting in loss of efficiency.
For rare earth optical centres, higher concentrations (e.g. densities of greater than 4% for Tb, or less for other rare earth species) give rise to quenching of optical centres due to cross relaxation and clustering effects (J. Sun et al., J. Appl. Phys. 97, 123513 (2005)).
Thus, problems with known device structures and processes based on luminescent centres comprising rare earths and/or nanocrystals include inconsistent size, quality and uniformity of nanocrystals to obtain a desired wavelength of emission or excitation energy; quenching of emission from rare earth luminescent species at higher concentrations, and poor efficiency of excitation of luminescent centres either directly or by energy transfer from nano-particles to rare earth luminescent centres. In particular, energy mismatches lead to poor excitation efficiency, i.e. if the excitation energy is too low, luminescent centers are not effectively excited, and if the excitation energy is too high, then energy is wasted in the excitation process.
It is desirable to overcome the above mentioned limitations and further improve the performance and efficiency of light emitting devices, particularly those based on silicon and other Group IV materials, which are compatible with silicon process technology, so as to provide alternatives to known solid state light emitting devices, such as those based on conventional direct band gap II-VI or III-V materials.
Thus, there is a need for alternative or improved materials, structures and/or methods of fabrication for solid-state light emitters, particularly for applications requiring higher brightness, luminous efficacy and reliability, such as solid state lighting.